1. Field of the Invention
The invention relates to data processing systems in general and more particularly to the detection of the idle link state in a communications subsystem.
2. Description of the Prior Art
Highly flexible and cost effective communications subsystems have been provided for coupling data processing systems to communication channels, such as those associated with a publicly accessible (e.g. telephone) communications network. Such systems have been embodied in hardware/firmware architectures which respond to commands from a communication processor associated with a central processing unit and enter into either a receive mode, a transmit mode, or concurrent transmit/receive modes for transferring data messages between the communication processor and the communication channel. Data transfers occur under the control of a firmware system acting in concert with a microprocessor within the adapter to assemble and disassemble whole or partial data bytes of varying bit sizes. System architectures which have been used readily accommodate an expansion of capacity and exhibit dynamic flexibility.
However, present communication line adapter systems are not able to provide the control functions required for interfacing with a synchronous communications network such as, for example, has been specified by the International Telegraph and Telephone Consultative Committee (CCITT) and described in the "CCITT Sixth Plenary Assembly-Orange Book, Vol. VIII.2 Public Data Networks", published in 1977 by the International Telecommunication Union, Geneva. Various specifications, in particular those designated X.21, X.24 and X.27 promulgated by the CCITT call for generation within the data network of bit and byte timing signals which are used for establishing a byte control protocol (BCP) or a bit oriented protocol (BOP) for organizing and handling data messages.
In a communication environment such as X.21, it is necessary to use different types of protocols (i.e., byte control protocol and bit oriented protocol) in order to send and/or receive messages with other data communication equipment. During the bit oriented protocol, the input line is said to be in an idle state when 15 or more consecutive binary ONE bits are received by the communications controller. This is not the case with the byte control protocol. The prior art such as described in U.S. application Ser. No. 53,110 entitled, "Communication Line Adapter for a Bit and Byte Synchronized Data Network" now abandoned, and U.S. application Ser. No. 911,635 entitled "Hardware/Firmware Communication Line Adapter", now U.S. Pat. No. 4,254,462, having the same assignee as the instant application, describe a system whereby the idle link state is controlled through software and firmware routines.
The prior art used a counter which operated under hardware and firmware control during the bit oriented protocol to monitor data received from the external communication devices and report the idle link state. Communication software then took appropriate action such as terminating the operation of the communication line or sending commands to re-establish the communication link.
With the advent of new communication procedures such as X.21, which use a combination of both bit oriented and byte control procedures, the requirement for firmware/software monitoring of the data resulted in excessive subsystem overhead thereby limiting the throughput of the communications subsystems.